This has been a weird issue for quite awhile. Again, I'm not worried about bugs like this with V3 as we're migrating to V4 soon which is a complete re-write of the software.
Well anyway I finally worked out the situation when this occurs.
I assume it's when a new topic gets approved by a mod, it shows that mod's pic. It only seems to happen in places where new topics need to be approved, and as a members first post. Of course I can't verfiy this because I don't know which mod has approved which topics :p
Jackus, the other site has the same issue (RMVXA). I always see Galv and Rikifive's pics no matter who posted.
Yes that's what made me suspicious. Over there, there was a topic I was going to approve but Galv beat me to it, and my notifications came up with Galv's pic.
That was the first time it had happened over there (Prolly cos i'm only following the MV forum and I mostly approve em myself lol)